The present invention pertains to a composite wear pad (and methods of making the same) adapted for use in conjunction with articles used in severe operating environments wherein wear-resistance and the thickness of the wear resistant layer are important properties. More specifically, the present invention pertains to a composite wear pad (and methods of making the same) adapted for use in conjunction with articles used in such severe operating environments wherein the wear pad includes a wear-resistant hard particle-containing layer (a cladding layer) metallurgically bonded to a metal or alloy substrate wherein as an option, the substrate is weldable. Further, the hard particle-containing layer or cladding layer of the present invention has a thickness equal to or greater than 3 millimeters (mm). The thickness of the cladding layer of the invention can have a thickness that ranges between about 3 mm and about 20 mm. Still further in another range, the thickness of the cladding layer of the invention can have a thickness that ranges between about 5 mm and about 15 mm. Yet, further in another range, the thickness of the cladding layer of the invention can have a thickness that ranges between about 10 mm and about 20 mm.
Equipment that operates in severe operating environments has typically benefited through the use of components that exhibit improved wear-resistance. In such severe operating environments, wear-resistance and the thickness of the wear resistant layer are important properties to achieve acceptable operational life. In the absence of such properties like wear-resistance, and/or the thickness of the wear resistant layer, the equipment can experience reduced life, which is undesirable.
One way to improve the operational life of equipment is to apply a coating. In this regard, coatings are often applied to equipment subjected to harsh environments or operating conditions in efforts to extend the useful lifetime of the equipment. Various coating identities and constructions are available and typically are selected based upon the mode of failure to be inhibited. For example, wear resistant coatings of ceramics (e.g. alumina, titanium nitride, titanium carbide etc), metal matrix composites (e.g. tungsten carbide-metal matrix coating), wear resistant alloys (e.g. Stellite alloys, triboloys), diamond, or diamond-like carbon have been developed for metallic substrates wherein the coating is deposited on the substrate by physical vapor deposition, chemical vapor deposition, thermal spray, electroplating or electroless plating. While such coatings provide benefits, one issue with these coating applications is the limitation of the coating thickness. Typically the thickness of the above coatings is from several microns to hundreds of microns, which limits the operational life of the equipment with more allowable wear.
A further way to extend the operational life of equipment with more allowable wear is to apply thicker claddings to the articles or components subjected to harsh environments or operating conditions. Various cladding identities and constructions are available, and typically are selected based on the mode of failure to be inhibited. For example, wear resistant chromium carbide-metal, tungsten carbide-metal stellite alloy, and tribaloy alloy claddings with different welding techniques like tungsten inert gas welding, metal inert gas welding, plasma transferred arc, and laser cladding have been developed for metal and alloy substrates. These claddings can be used in a composite wear pad comprising the cladding and a metal or metal alloy substrate, but as described hereinafter, these claddings have drawbacks.
One way to extend increased operational life of equipment of large allowable wear is the use of a chromium carbide weld overlay wear pad. However, there are some drawbacks to this kind of wear pad. Chromium carbide in a steel-based or nickel-based alloy matrix has a lower abrasion resistance, in comparison with tungsten carbide-metal based cladding so that it may be unsuitable for some severe environment applications. In order to achieve a sufficient thickness, multiple welding passes are necessary, which increase manufacturing costs. The intense heat in a welding process can cause dilution of the backing steel substrate and degradation of the weld overlay layer, which are undesirable. Weld overlays are typically not uniform in composition and microstructure, and may contain defects such as segregation and voids. The as-deposited surface of a weld is rough and requires additional machining to achieve a sufficient surface smoothness.
Another way to provide a composite wear pad to extend the operational life of equipment of large allowable wear is tungsten carbide-metal cladding by PTA (Plasma Transfer Arc) or laser cladding or other welding processes like tungsten inert gas welding and metal inert gas welding. Drawbacks with these methods are similar to the drawbacks connected with the chromium carbide weld overlay. These drawbacks include the need to use multiple passes to achieve the necessary thickness. The intensive heat connected with the welding process may also result in damaging the substrate including dilution of the steel substrate at different level dependent on the welding technology and dissolving of steel substrate into the wear pad causing a reduction in the wear resistance. Using multiple passes to achieve a required thickness typically causes defects such as nonuniformity in composition and microstructure, e.g. segregation and voids between the joints of welding passes. The as-deposited surface of a weld is rough and often requires additional machining to achieve a sufficient surface smoothness.
Another way to extend the operational life of the equipment includes the use of a cemented tungsten carbide wear plate with or without a backing member. It has the advantage of being very wear resistant. However, the use of such a structure, i.e., a cemented tungsten carbide wear plate with or without a backing member, displays a number of drawbacks. These drawbacks include the size limitation to the lateral dimension of the carbide pads, which increase the labor cost for applying the carbide pads to the equipment. The brittleness of carbide causes easy chipping, cracking and even fracture of the wear plate. The cemented tungsten carbide wear plate with or without a backing member is not weldable, and therefore, does not have as wide application as if it were weldable. The cemented tungsten carbide wear plate is typically attached to the equipment by brazing, adhesive or mechanical locking, and size of the cemented tungsten carbide wear plate is restricted by the method of attachment. If brazing is used, the size of the cemented tungsten carbide wear plate is limited to a size of, for example, two inches by two inches. The brazed joint also represents a weak point for the article, for when exposed to excessive wear, can be washed and cause pop-off of the wear pads or other modes of premature failure. If the attachment is by adhesive, the application temperature for the article or equipment is also limited, typically below about 200° C. dependent on the adhesive being used. The cemented tungsten carbide wear plate attached to the equipment by adhesive typically has even lower bonding strength than brazing and thus has the tendency to pop off of the backing substrate which could cause a catastrophic failure of the article or equipment. The inherent weakness of adhesive can limit the effective use of a tungsten carbide wear plate.
Yet another way to provide the increased life of an equipment subject to wear is to use a high chromium cast iron material. Drawbacks connected with this material include the fact that it is not weldable, difficult to braze or machine. Further, it has a low wear resistance in comparison to hard composite coatings or claddings.
The following exemplary patent documents disclose various ways that persons have tried to improve wear-resistance.
In reference to the use of flexible cloth, U.S. Pat. No. 3,743,556 to Breton et al. discloses the use of one flexible cloth containing metal matrix alloy particles and another flexible cloth containing hard particles wherein the cloths are positioned one on top of the other on the surface of a substrate to form a laminate. The laminate is heated under various conditions (e.g., at a temperature equal to 1040-1080° C. in a hydrogen purge) to form a hard layer on the surface of the substrate wherein the thickness of the hard layer in several examples appears to be “slightly greater” than 30 mils or 60 mils or 0.090 inches. U.S. Pat. No. 3,864,124 to Breton et al. and U.S. Pat. No. 4,194,040 to Breton et al. each discloses using flexible sheets to form products wherein the thickness of the hard layer is on the order of 0.030 inches or less. U.S. Pat. No. 3,916,506 to Wolf appears to also use flexible sheets to form a hard layer wherein the thickness thereof appears to be between 0.005 inches to 0.06 inches. One process parameter comprises heating in a hydrogen atmosphere at a temperature of 975-1150° C. to decompose the PTFE binder and to flow the Ni-based alloy. U.S. Pat. No. 4,624,860 to Alber et al. discloses the use of flexible sheet to achieve a hard layer of 0.060 inches. One exemplary process comprises heating under vacuum at 1120° C. for 20 minutes. U.S. Pat. No. 5,164,247 to Solanski et al. discloses the use of flexible cloth to achieve a layer with a thickness of 0.060 inches. One exemplary process comprises heating at 1140° C. for 30 minutes. Other patents that disclose the use of flexible cloth include U.S. Pat. No. 4,685,359 to Worthen et al., U.S. Pat. No. 5,236,116 to Solanski et al. and U.S. Pat. No. 5,352,526 to Solanski et al.
In reference to the use of a paint or paste, U.S. Pat. No. 3,779,715 to Wendler et al. discloses a two-step process to make a hard member. The first step comprises applying a paste of braze and hard particles to the surface of a mold and heating (at 1020° C. for 15 minutes) the same to form a skeleton comprising hard particles brazed together with a brazing alloy. The second step comprises infiltrating (in a dry hydrogen furnace at 982° C. for about 15 minutes) braze material into the skeleton to form the end product. The examples show thicknesses of 5 mm to 6.5 mm, and Example 6 appears to have a thickness equal to 13 mm. U.S. Pat. No. 6,649,682 to Breton et al. appears to use a paint system to achieve a hard layer with a thickness of greater than 6 mm. One exemplary heating process comprises heating at 350 Celsius per hour to 980° C. and holding for one hour, and then increasing the temperature at a rate of 180 Celsius per hour to 1120° C. and holding for 30 minutes.
In reference to infiltration, U.S. Pat. No. 4,017,480 Baum discloses infiltration of hard particles with an alloy. The infiltration is carried out in a hydrogen furnace at the brazing temperature of the alloy (e.g., 1150-1190° C. or 1065-1200° C. or 1100-1150° C.) for about 20 minutes.
Although the above coatings and claddings have been able to improve or prolong operating life of the articles or equipment used in such severe operating environments, there remains the need to provide an improved cladding that displays improved wear-resistance. There remains the need to provide an improved composite wear pad with thicker wear resistant cladding layer adapted for use in conjunction with articles used in such severe operating environments wherein the wear pad includes a wear-resistant hard particle-containing layer (a cladding layer) secured (e.g., metallurgically bonded) to a metal or metal alloy substrate wherein the composite wear pad displays improved wear-resistance.